1. Field of the Invention
This invention relates to a projector.
2. Related Background Art
A liquid crystal light value as the image forming means of a projector is often of the twist nematic type (hereinafter referred to as the TN type. This TN type liquid crystal, as is well known, is great in field angle characteristic and considerably differs in contrast depending on the direction in which it is seen (see Appl. Phys. Lett. 38 (1981), 497). Therefore, in a direct view liquid crystal display device wherein a liquid crystal light value is directly observed, the directions of liquid crystal molecules, a polarizer and an analyzer are adjusted in accordance with this characteristic.
Referring to FIGS. 1A and 1B of the accompanying drawings which shows this situation, the visual angle 44 of the observer 42 is inclined by an angle .theta. with respect to the normal 43 to a direct view liquid crystal display device 41 shown in FIG. 1A, and usually the area around this visual angle .theta. is the view field area. Accordingly, in order to adjust the field angle characteristic of the TN type liquid crystal to this area, use is made of a construction as shown in FIG. 1B wherein the directions of transmission polarization of a polarizer 45 and an analyzer 49 and the directions of orientation 46 and 48 of liquid crystal molecules 47 at the opposite ends of the liquid crystal layer thereof are inclined by 45.degree. with respect to a horizontal axis.
In a projector, the angular expanse of a light beam incident on liquid crystal is smaller than in the direct view type, and also for an improvement in the quality of image (particularly contrast), a similar construction is often used and there is also obtained a secondary effect such as a reduction in cost by production facilities being made common.
Also, in recent years, various polarizing illumination devices for converting indefinite polarized light from a light source into linearly polarized light having a particular direction of polarization have been proposed as means for enhancing the luminance of a projector and increasing the efficiency of light utilization of the projector, but for the reason set forth above, the linearly polarized light emitted from those polarizing illumination devices must have its direction of polarization inclined by 45.degree. with respect to said horizontal axis.
As a method of inclining the direction of polarization of linearly polarized light emitted from a polarizing illumination device, there are conceived polarizing conversion systems as shown, for example, in FIGS. 2 and 3 of the accompanying drawings. In FIG. 2 which shows only the essential portions of a polarizing illumination device described in Japanese Laid-Open Patent Application No. 61-90584, indefinite polarized light from a light source (not shown) is divided into two linearly polarized components S and P by the multi-layer film 1001 of a polarizing beam splitter, and the polarized component S is bent in the same direction of travel as the polarized component P by the total reflection surface 1002 of a rectangular prism, whereafter it has its direction of polarization rotated in the same direction of polarization as the polarized component P by a half wavelength optical phase plate 1003a. The two light beams which have been made to have the same direction of travel and the same direction of polarization in this manner are caused to enter a half wavelength optical phase plate 1003b, whereby the directions of polarization of the two light beams can be inclined in a direction depending on the optical axis of the half wavelength optical phase plate 1003b.
FIG. 3 shows an example in which quarter wavelength optical phase plates are used instead of half wavelength optical phase plates. This example is the same as the example shown in FIG. 2 in that the polarized component S is bent in the same direction of travel as the polarized component P by the total reflection surface of the rectangular prism 1002, but a quarter wavelength optical phase plate 1112a is disposed on the optical paths of two light beams so that said two light beams may become circularly polarized lights and further, a quarter wavelength optical phase plate 1112b is disposed so that said two circularly polarized light beams may become linearly polarized lights.
FIG. 4 of the accompanying drawings schematically shows a construction in which the polarizing illumination device shown in FIG. 2 or 3 is applied to a color projector. The reference numeral 31 designates a polarizing element shown in FIG. 2. White linearly polarized light emitted from the polarizing element 31 is resolved into three colors, red, green and blue, by a dichroic mirror 32 reflecting red and transmitting green and blue therethrough, a dichroic mirror 33 reflecting blue and transmitting green therethrough and a total reflection mirror 34, and the respective lights are transmitted through liquid crystal light valves 7R, 7G, 7B and polarizing plates 8R, 8G, 8B, and thereafter are again synthesized by a total reflection mirror 35, a dichroic mirror 36 reflecting blue and transmitting red therethrough and a dichroic mirror 37 reflecting green and transmitting red and blue therethrough.
The synthesized light is projected onto a screen, not shown, by a projection lens 10.
Accordingly, in this color projector, not only the efficiency of light utilization can be increased, but also the direction of polarization of the polarized illuminating light can be adjusted to the direction of orientation of liquid crystal molecules.
The system shown in FIG. 4, however, suffers from the following problems. The optical phase plate exhibits wavelength dependency and therefore, when an attempt is made to change the direction of polarization of light of a wide band like white light into a certain state, if for example, the optical phase plate is designed for the wavelength of the G component of white light, it will become impossible to shift the phase by the same amount as for the G component, for the B and R components having wavelengths differing from the wavelength of the G component. Accordingly, almost all part of the G component has its direction of polarization set to a predetermined state, while considerable parts of the B and R components have their directions of polarization not set to this state.
The polarizing conversion system is a system for supplying light having a particular direction of polarization and thus, light which does not have this direction of polarization is not utilized. Accordingly, considerable parts of the B and R components are losed due to the wavelength dependency of the optical phase plate and moreover, the light from the projector becomes greenish. Also, for a similar reason, considerable parts of the G and R components will be losed if the optical phase plate is designed for the wavelength of the B component of white light, and considerable parts of the B and G components will be losed if the optical phase plate is designed for the wavelength of the R component of white light.